Conventionally, foot pedal controls such as the brake or accelerator pedal in a motor vehicle are connected to the corresponding vehicle component through a cable or other mechanical linkage. However, it may be desirable for a foot pedal to control the corresponding engine component such as the engine or brakes via an electrical signal. In order to achieve this, it is necessary to produce an electrical signal representative of the instantaneous position of the foot pedal.
Position sensors are known that use a potentiometer for detecting the position of a foot pedal. Such potentiometers rely on a sliding contact, and use of such a contact may result in wear and abrasion which could change the characteristics of the sensor over a period of time.
According to one aspect of the invention, there is provided a position sensor for sensing the relative position of two components, the sensor comprising a stator connected to one component and having a surface which includes two magnetically separate regions of magnetic material separated by a channel; a magnetic unit connected to the other component and having north and south pole faces arranged relative to the surface so that when the magnetic unit is in a first position relative to the stator, at least a substantial part of one pole face lies over one of the regions of magnetic material and at least a substantial part of the other pole face lies over the other region of magnetic material, thereby producing a magnetic field across the channel, and when the magnetic unit is in a second position the north and south pole faces of the magnetic unit lie over the same region of magnetic material such that no magnetic field is produced across the channel; and a magnetic field sensor placed in the channel so as to produce a signal which is dependent on the magnetic field in the channel and which represents the relative position of the two components.
Because the position sensor does not rely on a sliding electrical contact, electrical problems due to wear and abrasion within the sensor will be alleviated and the reliability of the sensor can be improved.
When the sensor is used to sense the position of a pedal in a motor vehicle, the stator may be fixed to the vehicle body (the one component) and the magnetic unit may be fixed to the pedal (the other component). Movement of the foot pedal then causes the magnetic unit to move relative to the stator, and the position of the foot pedal can be determined from the sensed position of the magnetic unit.
The signal produced by the magnetic field sensor could be passed to the engine control of the vehicle for example, thus replacing the conventional mechanical connection between the engine and the accelerator pedal.
In most applications the stator is held stationary whilst the magnetic unit is moved, but it is also possible for what is here termed the stator to be moved whilst the magnetic unit is held stationary.
The magnetic unit may be made entirely of a permanent magnet material, but preferably the magnetic unit comprises a permanent magnet fixedly attached to a section of magnetic material with at least one pole face of the magnetic unit being formed by the magnetic material. The section of magnetic material may be made from mild steel so that it can easily be machined into the desired shape.
To allow the stator to be mounted on an electronic circuit board, the regions of magnetic material may be held in their correct positions by being attached to a non magnetic base.
The stator surface and the pole faces are preferably flat, the pole faces being able to move in a plane that is adjacent to the surface of the stator, the separation between the stator surface and the pole surfaces being small to ensure a good magnetic coupling between the magnetic unit and the stator.
To allow free movement of the pole faces relative to the stator surface, a layer of plastics material such as Teflon may be placed between the stator surface and the pole faces.
The magnetic unit is preferably arranged so that as it moves from the first position to the second position, one pole face remains over a single region of magnetic material whereas at least part of the other pole face crosses from one region of magnetic material to the other region. This arrangement is useful because when one pole face lies above two separate regions of magnetic material, continuous displacement of that pole face results in a continuous change in the relative coupling to each magnetic region, and hence a continuous change in the magnetic field within the channel.
Although the channel may be a single straight line, the channel preferably comprises a subsidiary channel section and a main channel section at an angle to one another, the magnetic unit being arranged such that movement of the magnetic unit is in a direction parallel to the subsidiary channel. This conveniently allows both poles of the magnetic unit to be moved together along the direction followed by the subsidiary channel such that only one of the poles crosses the main channel, the other pole remaining over the same region of magnetic material.
To help ensure the position sensor provides a linear position signal, the incremental area of a pole face crossing the main channel per unit displacement of the magnetic unit may be constant throughout the sensed traveling distance of the magnetic unit. To help further ensure the linearity of the position sensor, the leading and trailing edges of the pole face crossing the main channel may be parallel to the main channel as they cross the main channel.
The magnetic field sensor may reside in either the subsidiary channel section or the main channel section, but preferably the magnetic field sensor will reside in the main section, orientated to measure the magnetic field component perpendicular to the length of the main channel section.
In a preferred embodiment, the subsidiary channel section and the main channel section form a right angle where they meet, the channel and the magnetic unit being arranged so that when the magnetic unit is in the first position, each pole face resides on a different side of the subsidiary channel, thus producing a magnetic field that is substantially perpendicular to the subsidiary channel.
With the magnetic field sensor placed in the main channel, this arrangement allows for a measurement of the magnetic field component due to the magnetisation of the stator whilst reducing the contribution from the magnetic field component produced directly by the magnetic unit, thereby alleviating the proximity effects of the magnetic unit as it passes over the channel and further improving the linearity of the position sensor.
To further reduces the proximity effects of the magnetic unit, the main channel may have a small width in comparison to its depth.
The subsidiary channel may be straight, but in a preferred embodiment the subsidiary channel is arcuate and the magnetic unit is connected to an arm pivoted at 90 degrees to the stator surface, so that movement of the arm causes the magnetic unit to move over the stator surface along the subsidiary channel.
The arm may be pivoted in common with a foot pedal such that angular displacement of the foot pedal causes an identical angular displacement of the magnetic unit relative to the stator, thereby allowing the position of the foot pedal to be determined from the position of the magnetic unit.
To facilitate fabrication, the arm may be made of magnetic material and formed integrally with the magnetic unit.
The magnetic field sensor may conveniently comprise at least one Hall probe, preferably producing a Hall voltage proportional to the sensed magnetic field. Since a Hall probe can be formed from a thin layer of electrically conducting material which senses the magnetic field perpendicular to the layer, the Hall probe may conveniently be placed within a narrow channel to measure the field across the width of the channel.
In a simple embodiment, the channel has parallel vertical side walls formed by the regions of magnetic materials, the floor of the channel being formed by the non magnetic base.
However, in another embodiment, at least the main channel section when viewed in cross section may have a recess parallel to the stator surface, the recess being formed by an area of overlap between the two regions of magnetic material. Such an arrangement allows a Hall probe to lie parallel to the stator surface when mounted in the channel, thereby facilitating the incorporation of the position sensor into a circuit formed on a circuit board.
The invention also extends to a foot pedal assembly for use in a motor vehicle, the assembly comprising a pedal base to be fixedly mounted in a motor vehicle, a pedal pivotably mounted on the base, and a position sensor as claimed in any preceding claim arranged to sense the angular position of the pedal relative to the pedal base.
According to another aspect of the invention, there is provided a magnetic field sensor comprising a first and a second electronic chip, each chip having a flux sensitive area for sensing magnetic field and an integrated circuit integrally formed therewith such that each chip produces an electrical output signal representative of the magnetic field sensed by each respective flux sensitive area, wherein the first and second chips are mounted adjacent to one another in a single package such that the separation between the flux sensitive area of each chip is 1 mm or less.
The two signals produced by each chip can be compared with one another to ensure that the sensor is working correctly. Because the two flux sensitive regions are close together, any spacial non uniformity in the magnetic field will produce only a small difference between the output signal of each chip.
The separation between the flux sensitive regions will be understood to be the separation between the closest points of these regions.
The two chips may be integrated onto a single piece of semiconductor material, in which case the separation between the two flux sensitive regions will preferably be less than 500 microns.